In a hydraulic excavator, front members such as a boom are operated by an operator manipulating respective manual control levers. However, because the front members are coupled to each other through articulations for relative rotation, it is very difficult to carry out excavation work within a predetermined area by operating the front members. In view of the above, area limiting excavation control systems are proposed in JP, A, 8-333768, WO 95/30059 and WO 95/33100, aiming to facilitate such excavation work.
The area limiting excavation control system for a construction machine disclosed in JP, A, 8-333768 comprises a multi-articulated front device made up of a plurality of front members being rotatable in a vertical direction, a plurality of hydraulic actuators for driving the plurality of front members, a plurality of operating means for instructing operation of the plurality of front members, and a plurality of hydraulic control valves driven upon manipulation of the plurality of operating means and controlling respective flow rates of a hydraulic fluid supplied to the plurality of hydraulic actuators, wherein the control system further comprises area setting means for setting an area where the front device is movable; first detecting means for detecting status variables relating to the position and posture of the front device; first calculating means for calculating the position and posture of the front device based on signals from the first detecting means; second calculating means for calculating the speed of the front device which depends on driving of at least a first particular actuator associated with a first particular front member among the plurality of hydraulic actuators; third calculating means for receiving values calculated by the first and second calculating means and calculating, based on the received values, a limit value of the speed of the front device which depends on driving of at least a second particular actuator associated with a second particular front member among the plurality of hydraulic actuators so that when the front device is inside the set area near the boundary thereof, the moving speed of the front device in the direction toward the boundary of the set area is restricted; and signal modifying means for modifying an operation signal from the operating means associated with the second particular actuator so that the speed of the front device which depends on driving of the second particular actuator will not exceed the limit value. When the front device is inside the set area near the boundary thereof, the third calculating means calculates a limit value of the speed of the front device which depends on driving of the second particular actuator associated with the second particular front member, and the signal modifying means modifies an operation signal from the operating means associated with the second particular actuator so that the speed of the front device which depends on driving of the second particular actuator will not exceed the limit value. Therefore, direction change control is carried out in such a manner as to slow down motion of the front device in the direction toward the boundary of the set area, enabling the front device to be moved along the boundary of the set area. It is hence possible to smoothly and efficiently perform excavation with the boundary of the set area set as a target excavation plane, while a bucket is kept from moving out beyond the boundary of the set area, i.e., the set depth of excavation.
According to the area limiting excavation control system disclosed in WO 95/30059, an area where a front device is movable is set beforehand. A control unit calculates the position and posture of the front device based on signals from angle sensors, and also calculates a target speed vector of the front device based on signals from control lever units. When the front device is inside the set area not near the boundary thereof, the target speed vector is maintained as it is. When the front device is inside the set area near the boundary thereof, the target speed vector is modified to reduce a vector component in the direction toward the boundary of the set area. Hydraulic control valves are then operated so that the modified target speed vector is obtained. As a result, excavation in a limited area can efficiently and smoothly performed.
According to the area limiting excavation control 1system disclosed in WO 95/33100, in consideration of the fact that the metering characteristic of a hydraulic control valve I(flow control valve) changes depending on the load of a front device in the control system disclosed in WO 95/30059, the function relationship used in a target pilot pressure calculating portion is modified in accordance with change in load of the metering characteristic of the flow control valve, and a target pilot pressure is calculated using the modified function relationship. Highly accurate control can be thus achieved regardless of load change so that a tip of the front device moves as per the target speed vector.